In a vehicle such as a car, a transmission is interposed between an internal combustion engine and a driving wheel. The transmission changes a driving force applied to and a travel speed of the driving wheel to meet travel conditions of the vehicle changing within a wide range, so as to optimize the performance of the internal combustion engine.
As this transmission, there are known a gear type transmission which changes gear ratios in discrete steps by selective switching of the engaged states of plural steps of gear trains, and also a continuously variable transmission for increasing and decreasing the width of a groove formed between the pulley parts of a driving side pulley and a driven side pulley which each have a fixed pulley part fixed to a rotating shaft and a movable pulley part supported on the shaft for movement toward and away from the fixed pulley part to thereby increase and decrease the rotating radius at each pulley of a belt wound on the pulleys to thereby allow a continuous change of a belt ratio for transferring the driving force. Continuously variable transmissions of this type are disclosed for instance in Japanese Patent Laid Open Publication Nos. 186656/1982, 43249/1984, 77159/1984, and 233256/1986.
In the conventional continuously variable transmission, an arrangement controls an actual belt ratio to be a target belt ratio determined by a speed change schedule map based on the throttle opening and the number of engine revolutions, or controls the actual number of engine revolutions to be a target number of revolutions determined by a change schedule map based on the throttle opening and vehicle speed. In the continuously variable transmission controlling the actual number of engine revolutions toward a target number, it is conventional that, when the target number of engine revolutions is changed according to a change in the travel mode or the throttle opening during travel of the vehicle, the filtered target number of engine revolutions (NESPF) obtained by applying a first order lag filter to the target number of engine revolutions (NESPR) is defined to be the final target number of engine revolutions (NESPRF), and then the actual number of engine revolutions (NE) is controlled to be such final target number of engine revolutions (NESPRF), as shown in FIG. 6.
In this case, the values of the filter are set separately for the characteristics of respective travel modes such as an economy travel mode (ECN), a power travel mode (POW), and a low travel mode (LOW). Namely, respective values of the filter are used for the economy travel mode (ECN) in the case of travelling on a flat road while maintaining a low fuel consumption, for the power travel mode (POW) in the case of sporty travel or adverse road travel, or for the low travel mode (LOW) in the case of travel on a slope or the like requiring the transfer of a high torque or engine braking.
However, in the conventional continuously variable transmission revolution number controller, when the travel mode is changed or when the throttle opening is changed to a completely opened state during vehicle travel, the target number of engine revolutions (NESPR) is changed, as shown in FIG. 6, and causes the filtered target number of engine revolutions (NESPF) to be changed. In this case, a problem has been produced because the filtered target number of engine revolutions (NESPF) is changed at the highest rate of change immediately after the target number of engine revolutions (NESPR) is changed, which causes the actual number of engine revolutions (NE) to be abruptly changed.
Therefore, there is the inconvenience that the actual number of engine revolutions (NE) is quickly changed when the target number of engine revolutions is changed during travel of the vehicle, thereby giving to the driver an uneasy feeling as if the vehicle had slipped due to an incomplete transfer of the driving force. Further, there has been another inconvenience in that, since power is applied by the change in the actual number of engine revolutions (NE), it is difficult to control the belt ratio after the change of the target number of engine revolutions (NESPF). Further, there has been another inconvenience in that the change in the actual number of engine revolutions (NE) does not cause the power performance to be improved, but instead a shock or a noise is generated in case of a rapid shift down due to a quick opening of the throttle, thereby giving an uneasy feeling to the driver.
In order to overcome these inconveniences, one conventional approach was to set the value of the filter to a low value to slow the change in the number of engine revolutions, but of course the time required to reach the new target number of engine revolutions was lengthened, whereby the system response characteristic deteriorated and a delay to carry out a movement different from a driving operation was noticed at the same time, thereby giving an uneasy feeling to the driver.
A purpose of the present invention is to provide a revolution controller for a continuously variable transmission which prevents the actual number of engine revolutions from being rapidly changed when the target number of engine revolutions is changed according to a change in travel mode or a change in the throttle opening during travel of the vehicle, in particular by limiting the rate of change of the number of engine revolutions to be lower in some cases than a prescribed rate, thereby avoiding giving an uneasy feeling to the driver, improving power performance, reducing the generation of shock or noise due to a quick opening of the throttle, and improving the response characteristic.